The disclosure relates to an extruder having a housing comprising at least two conveyor shafts with mutually wiping conveyor cross sections that can be driven in the same direction and are disposed in at least two housing bores having a cylindrical or conical design, the axes thereof being disposed parallel to or at an angle to each other. The distance between the axes of the two housing bores is greater than D/1.4142, where D is the diameter of the cylindrical bore or the greatest diameter of the conical bore. A clearance is provided between the housing bores and the outer diameter of the conveyor shafts.
Auger shafts rotating in the same direction, which can be optimally equipped with dual conveyor elements in an extruder, are used for continuously feeding, mixing, dispersing, and extruding plastics in particular, and in continuous mechanical process engineering. The ability to process as many products as possible with one auger configuration, with no loss of quality, intermediate cleaning, or machine stoppage is of great economic significance. There is a very close relationship between the dwell time behavior of the product in the system and the material exchange and self-cleaning in the processing area. A wide process window means diverse utilization and product variety.
Double augers having a modular conveyor element system have long been known. For the geometric design of tightly meshing conveyor systems, according to the state of the art, the clearance between the core diameter of the conveyor element and the flight of the meshing conveyor element is typically 1.0% of the housing bore, and between the flight and the bore is typically 2 to 10%. For unfavorable circumstances, mainly for larger diameters and/or products having very different particle sizes, according to DE 10 2008 029 305 AI, the flight angle is reduced by means of a profile correction in order to reduce the specific energy input and thus partial product damage and the risk of machine damage.
An extruder as described above is previously known from DE 10 2004 010 553 AI and is particularly suitable for sensitive materials and/or high shaft speeds due to the increased clearance between the housing bore and the flight of the conveyor shafts. Because the housing bore cannot be scraped off, however, the field of application of the known extruder is limited for screw and for kneading blocks.
The object of the invention is to improve product quality and reduce torque load peaks on the support shafts, thereby decisively improving the economic efficiency of the extruder. The object is achieved according to the invention by providing the extruder with each conveyor shaft having a plurality of conveyor segments and the offset angle of at least two adiacent conveyor segments on each conveyor shaft relative to each other is 180°, wherein the center point of the cross sectional profile of each conveyor segment is offset eccentrically relative to the center point of the housing bore and the center of rotation of the conveyor shaft, wherein the center point of the cross sectional profile of each conveyor shaft is formed by the intersection of the two mirror planes of the cross sectional profile of the conveyor shaft.
The extruder according to the invention comprises at least two double-fluted conveyor shafts comprising a precisely mutually wiping conveyor cross section, that is, the clearance substantially only compensates for production tolerances.
For the augers of a double auger extruder comprising a geometry precisely wiping the bore wall, according to the book “The co-rotating double auger extruder”, by Klemens Kohlgrüber, Carl Hanser Verlag, Munich 2007, Page 102, the formula
            A      /      D        ≥          cos      ⁡              (                  90          Z                )              ,applies, where
A is the distance between the axes of the two bores, D is the bore diameter, and Z is number of flutes, that is, for double-fluted augers having a geometry precisely wiping the cross sectional profile of the auger shafts and the bore wall, A/D>cos 45°>0.707. That is, the distance A between the axes of the two bores is greater than D/1.4142.
Each conveyor shaft comprises at least two adjacent conveyor segments. The angle of rotation from one conveyor segment of the at least two conveyor segments to the adjacent conveyor segment is 180°, that is, the at least two adjacent conveyor segments on each conveyor shaft are rotated 180° relative to each other.
The conveyor cross sections of the conveyor shaft segments wiping each other are rotated 360°/n to each other, where n is number of flutes.
The extruder according to the invention can, as is typical, comprise cylindrical bores having parallel axes. The invention, however, further relates to extruders having conical bores comprising axes approaching each other at an angle.
The center of the cross sectional profile of each conveyor segment is disposed eccentrically relative to the center of the housing bore and the center of rotation of the conveyor shaft. The center of the cross sectional profile of each conveyor segment is formed by the intersection of the two mirror planes of the cross sectional profile of the conveyor segment.
The eccentricity by which the center of the cross sectional profile of the conveyor segments is offset relative to the center of the housing bore and the center of rotation of the conveyor shafts is no greater than (D−A)/4.
The center of the cross sectional profile of the conveyor segments is preferably offset by 45° relative to the two mirror planes of the cross sectional profile, because at this angle all bores are wiped in every angular position of the shafts.
For a double-fluted conveyor shaft, one flute of each of the conveyor segments thereby comprises a clearance from the housing bore corresponding to the eccentricity, while the second flute of the conveyor shaft continuously wipes the housing bore. Both the housing bore and the conveyor profile of the auger shafts are thereby wiped along the entire circumference.
The arbitrary selection of the eccentricity between the housing bore diameter and the outer diameter of the conveyor shaft, and thus the layer thickness, makes the flight depth correspondingly less.
The reduced conveyor effect due to the gap between the conveyor shaft and the housing bore causes the fill level and material exchange to increase, while the partial material loading is reduced.
According to the invention, the flight of the one conveyor segment of each conveyor shaft wipes against the bore wall, while a gap is formed between the adjacent flight of the adjacent conveyor segment of the corresponding conveyor shaft. The flow direction of the product is thereby disturbed. That is, the material that is blocked against the flight wiping against the bore wall can flow out into the area of the gap formed by the adjacent flight of the adjacent conveyor segment of said conveyor shaft.
The two adjacent conveyor segments are preferably each formed by two partial discs of a cam disc, wherein both partial discs of each cam disc are rotated by 180° relative to each other.
By means of the extruder according to the invention, balanced dynamic conditions of the conveyor shaft are achieved, which are of great importance, as uniform loading serves as the basis for the technical design of the support shafts and the gearbox, and is a prerequisite for economical long-term operations. Measurements of the torque at the support shafts, using a resolution of milliseconds, have indicated that there are no further technical or technological requirements of typical kneading blocks that cannot be met. While the technical requirement for an overload of 10% for a maximum permissible long-term load on the shafts must be considered very critically, kneading blocks can often cause a load to vary by 30% for each revolution. In order to prevent fatigue failure of the shafts in all cases, the full machine rated power cannot be used. Even for a profile wrap of 180° of the individual discs in kneading blocks, a better product can be achieved under lower peak loads, and the 180° wrap is used for continuously running an auger thread, providing the least fluctuations of the peak load with a better product mix.
The length of each conveyor segment or each partial disc in the axial direction is preferably no greater than the bore diameter, particularly a maximum of half of the bore diameter.
Each conveyor shaft and the at least one conveyor element according to the invention supported thereby can form a single component. By implementing the shaft and conveyor element as one piece, a substantially greater torque can be transmitted. For example, the shaft, including the drive shaft, and the conveyor element in the area of the filling zone, including the melt zone, can be made as a single component.
According to the invention, a repeated wound profile is used, providing additional resistance to the axial flow rate of the product. That is, the flight depth over the entire exposed cross section is changed, whereby the entire product and the extruder are uniformly more greatly loaded.
The flights of the conveyor shaft can thereby run perpendicular to the axes of the holes, or can be implemented diagonally as augers.
The conveyor shafts preferably comprise barrier rings concentric to the bore axis between two conveyor segments, the diameters thereof corresponding to no greater than the distance between the axes. The material flow in the axial direction can thereby be slowed, and the dispergent effect can be increased. The barrier rings can thereby be disposed at varying distances between the conveyor segments.
The conveyor segments can also be formed by segments of a one-piece conveyor element, obtained by milling a metal part, for example.
By means of the extruder according to the invention, tensile and compressive forces are exerted on the solid agglomerates present therein by means of the viscosity of the continuous phase, leading to the fatigue failure of the same.
Therefore, effective homogenization and dispersion of the solid is achieved according to the invention, even for solid particles having a particle size of less than 100μ, particularly less that 10μ. By means of the extruder according to the invention, the product is thus continuously exposed to a different flow direction and speed, and therefore different strain conditions.
According to the invention, the large distance from the one flight of the conveyor segments to the housing bore causes great material exchange, while only slight material exchange takes place where the flight wipes against the wall of the housing bore. However, the greatest dispergent effect can be achieved there by highly loading the product, as well as by the precisely wiping conveyor cross sections of the two conveyor shafts. Due to the mutually wiping conveyor segment rotated by an angle of 180° at the one or the other conveyor shaft for a double-flute conveyor shaft, the product is subjected to external loading only in small areas, whereby the temperature of the product must be kept low overall, so that the product is not damaged, even in the extremely highly loaded areas, and is therefore handled gently overall. The extruder according to the invention is thereby particularly suited for processing sensitive products, such as for producing polyethylene terephthalate (PET).
According to a preferred embodiment, the extruder comprises at least six conveyor shafts disposed along a circle at equal central angle spacing, each wiping against the two adjacent conveyor shafts. The vacuum is thereby doubled with particularly high density at the flows. Because this is done without peak loads and thereby without an undesirable drop in viscosity over the entire length of the conveyor shafts, an effective, rapid mixing is achieved, as well as even substantially higher product quality and greater throughput with uniform homogenization.
If the conveyor cross sections of the conveyor shafts for the said embodiment are formed by conveyor segments wherein the conveyor segments wiping each other for a double-fluted conveyor shaft are rotated by 180°, the profile wrap, that is the stepwise change in flight depth in the axial direction between two adjacent shafts, is doubled and thereby substantially increased.